Among existing standards of wireless power transfer, the Rezence standard developed by Alliance for Wireless Power (A4WP, now AirFuel Alliance) is promising in multiple-receiver support and high placement freedom. To comply with the Rezence standard, the power amplifier which drives the transmitter coil needs to switch at 6.78 MHz efficiently and reliably. As a result, zero-voltage switching (ZVS) should be guaranteed for all the power semiconductor switches in order to reduce or eliminate switching losses. Traditional power amplifiers for wireless power transfer only accept a low voltage (e.g., <100 V) DC input so an additional power conversion stage is needed to interface with the 60 Hz AC utility. This multi-stage approach reduces overall power transfer efficiency and increases cost.
An alternative solution is to directly convert the 60 Hz AC voltage to drive the transmitter coil at 6.78 MHz. By combining the multiple stages, this single-stage power amplifier can potentially achieve both higher power transfer efficiency and low system cost. The amplifier should switch higher voltage (e.g., up to about 400V) rectified from AC line at the same 6.78 MHz. As switching losses scale up with the square of the switching voltage, the power amplifier will suffer from overheating if those losses are not significantly reduced or eliminated. Therefore, zero-voltage switching becomes even more advantageous for a single-stage power amplifier.